In view of a recent increase in concerns about the global environment, there have been proposed a number of electric vehicles, such as battery powered vehicles and fuel cell powered vehicles, which rely on an electric motor instead of an internal combustion engine as the drive source.
In the electric vehicles proposed to date, the structure contemplated to replace the conventional power unit that includes an internal combustion engine is simply a modified drive unit that includes a drive motor. Consequently, mounting systems for mounting the drive unit on the vehicle body, such as that disclosed for example in JP-A 7-156663 (Patent Document 1), have retained generally the same construction as mounting systems for conventional power units that include an internal combustion engine.
However, research conducted by the inventors led to the conclusion that where a drive unit vibration damping support for an electric vehicle has generally the same structure as a conventional mounting unit for a internal combustion engine, it is extremely difficult to achieve the vibration damping characteristics desired in an electric vehicle. A possible technical reason is that an electric motor and an internal combustion engine differ not only in terms of their construction, but also significantly in terms of their output characteristics, and thus there is considerable divergence in relation to their required characteristics, as well as the vibration damping support that would be appropriate for achieving these.
As a specific example, an electric motor develops high torque in a low speed range, whereas an internal combustion engine develops high torque in a high speed range, and thus there is a great difference in the characteristics of the drive torque reaction force to which the vibration damping support is subjected. Additionally, whereas vibration damping of idling vibration when the vehicle is at an idle is a requirement of mounting systems for internal combustion engines, the drive unit of an electric vehicle lacks an idling condition, and thus there is no need for a vibration damping support for such a unit to provide vibration damping of idling vibration. Moreover, in an electric motor, depending on the number of magnets and coils, vibration caused by output torque fluctuations may occur several times to several tens of times during each one revolution of the output shaft of the electric motor, whereas in a four-cycle internal combustion engine, vibration does not occur more than once during each two revolutions of the crankshaft. For this reason, the frequency of vibrations caused by torque fluctuations differ greatly, resulting in a major difference in the vibration damping characteristics that are required. The technical basis for the inventors' finding that support structures for conventional power units containing an internal combustion engine are inappropriate for use as support structures for drive units containing an electric motor may be understood in consideration of the above findings.